Ion Stratification Using Bias Pulses of Short Duration

ABSTRACT

A plasma processing apparatus includes a plasma processing chamber configured to contain a plasma comprising a plasma sheath, ions of a first species, and ions of a second species, a substrate disposed in the plasma processing chamber, and a short pulse generator coupled to the substrate, the short pulse generator configured to generate a pulse train of negative bias pulses. Each of the negative bias pulses has a pulse duration less than 10 μs. A pulse delay between successive negative bias pulses is at least five times the pulse duration. The first species has a first mass and the second species has a second mass less than the first mass. The pulse train spatially stratifies the ions of the first species and the ions of the second species in the plasma sheath.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 16/934,871, filed on Jul. 21, 2020, which application is herebyincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to plasma processing, and, inparticular embodiments, methods and systems for plasma processing thatspatially stratify species of ions in plasma using bias pulses of shortduration.

BACKGROUND

Device fabrication within microelectronic workpieces may involve aseries of manufacturing techniques including formation, patterning, andremoval of a number of layers of material on a substrate. There is aconsistent and continuous push to improve the fabrication processes,features, and capabilities of microelectronics. These improvements mayrequire new chemistry development as well as new advanced methods forprocess control.

Plasma processing is used in semiconductor device fabrication for manymanufacturing techniques, such as deposition and etching. Pulsed plasmaprocessing methods may utilize pulses of source power and/or bias powerto control various parameters during plasma processing. For instance,radio frequency (RF) power or direct current (DC) power may be pulsed.RF power may also be combined with a DC offset, such as when applyingbias pulses to an electrode.

The plasma may include various species mixed together within theprocessing chamber. Additionally, each species within the plasma maygenerate a variety of plasma products such as ions, radicals, electrons,and dissociation products. The plasma products of each species may havedifferent properties and be included for different purposes in theplasma. For example plasma products of different species may bedifferent chemical properties such as differing reactivity relative tovarious materials of a substrate being processed. Further, variousspecies within the plasma (and consequently the corresponding plasmaproducts) may have different masses, in addition to many otherdistinguishing features.

Control over individual species and species products within plasma maybe advantageous to maximize respective roles of the species duringplasma processing. For instance it may be desirable to control relativeflux rates of various plasma products at a substrate in order to furtheroptimize desirable parameters such as selectivity, etch profile,critical dimension, and others. Therefore, a novel plasma processingapproach for advanced process control of individual species and speciesproducts of the plasma may be desirable.

SUMMARY

In accordance with an embodiment of the invention, a method of plasmaprocessing includes generating plasma in a plasma processing chambercontaining a first species, a second species, and a substrate. Theplasma includes a plasma sheath, first species ions, and second speciesions. The first species has a first mass and the second species has asecond mass that is less than the first mass. The method furtherincludes applying a pulse train of negative bias pulses to thesubstrate. Each of the negative bias pulses has a pulse duration lessthan 10 μs and spatially stratifies the first species ions and thesecond species ions in the plasma sheath. No bias voltage is applied tothe substrate during a pulse delay after each negative bias pulse. Thepulse delay is at least five times the pulse duration.

In accordance with another embodiment of the invention a method ofplasma processing includes generating plasma in a plasma processingchamber containing a less-reactive species, a more-reactive species, anda substrate including an etchable surface, increasing the flux andenergy of ions of the less-reactive species at the substrate relative tothe flux and energy of ions of the more-reactive species at thesubstrate by applying a pulse train of negative bias pulses to thesubstrate, and etching the etchable surface of the substrate using theradicals of the more-reactive species. The plasma includes the ions ofthe less-reactive species, and the ions and radicals of themore-reactive species. The mass of the less-reactive species is lessthan the mass of the more-reactive species. The reactivity of theless-reactive species towards the etchable surface is less than thereactivity of the more-reactive species towards the etchable surface.Each negative bias pulse has a pulse duration less than 10 μs.

In accordance with still another embodiment of the invention a plasmaprocessing apparatus includes a plasma processing chamber, a substratedisposed in the plasma processing chamber, and a short pulse generatorcoupled to the substrate. The plasma processing chamber is configured tocontain a plasma including a plasma sheath, ions of a first species, andions of a second species. The first species has a first mass and thesecond species has a second mass less than the first mass. The shortpulse generator is configured to generate a pulse train of negative biaspulses. Each of the negative bias pulses has a pulse duration less than10 μs. A pulse delay between successive negative bias pulses is at leastfive times the pulse duration. The pulse train spatially stratifies theions of the first species and the ions of the second species in theplasma sheath.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1A, 1B, and 1C schematically illustrate an example plasma in thepresence of a bias electrode in accordance with an embodiment of theinvention, wherein FIG. 1A shows a wall sheath of the plasma interfacingwith the bias electrode when no voltage is applied, FIG. 1B shows amatrix sheath of the plasma interfacing with the bias electrode when apulsed voltage is applied, and FIG. 1C shows a Child-Langmuir sheath ofthe plasma interfacing with the bias electrode when continuous DCvoltage is applied;

FIG. 2 schematically illustrates an example plasma processing apparatusincluding a matrix sheath of a plasma interfacing with a substratewithin a plasma processing chamber, the substrate being coupled to ashort pulse generator in accordance with an embodiment of the invention;

FIG. 3 illustrates an example pulse train of negative bias pulses inaccordance with an embodiment of the invention;

FIG. 4 illustrates an example pulse train of negative bias pulse andpositive bias pulse in accordance with an embodiment of the invention;

FIG. 5 illustrates an example pulse train of negative bias pulses, eachwith a linear voltage slope in accordance with an embodiment of theinvention;

FIG. 6 illustrates an example continuous wave pulse train includingnegative bias pulses and positive bias pulses in accordance with anembodiment of the invention;

FIG. 7 illustrates an example modulated wave pulse train including asurface reaction phase followed by a chemical modification phase inaccordance with an embodiment of the invention;

FIG. 8 schematically illustrates an example plasma processing apparatusincluding a plasma coupling element used to generate a plasma, a firstgas source, a second gas source, and a substrate within a plasmaprocessing chamber, the plasma including a matrix sheath interfacingwith the substrate which is coupled to a short pulse generator inaccordance with an embodiment of the invention;

FIG. 9 illustrates an example method of plasma processing in accordancewith an embodiment of the invention; and

FIG. 10 illustrates another example method of plasma processing inaccordance with an embodiment of the invention.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale. The edges of features drawn in thefigures do not necessarily indicate the termination of the extent of thefeature.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of various embodiments are discussed in detailbelow. It should be appreciated, however, that the various embodimentsdescribed herein are applicable in a wide variety of specific contexts.The specific embodiments discussed are merely illustrative of specificways to make and use various embodiments, and should not be construed ina limited scope.

During plasma processing used in semiconductor device fabrication, avariety of species may be present within the plasma for a variety ofreasons. For example, an inert gas (e.g. used as a carrier gas) istypically mixed with one or more reactive gases with a plasma processingchamber. As a basic scenario, species A and species B mixed in theplasma processing chamber may be ionized in the plasma to producecorresponding positively charged ions such as ions A⁺ and ions B⁺ aswell as other ionized cracking patterns. Bias power may be applied at abias electrode, (e.g. to a substrate holder immobilizing a substrate) toaccelerate the ions toward the substrate.

When the bias voltage is applied at relatively large time scales (e.g.for a duration t greater than the reciprocal plasma frequency of theions: t»ω_(pi) ⁻¹), a steady state Child-Langmuir sheath with sheathvoltage V_(DC) may be formed. For instance, bias voltage with a DCoffset of V_(DC) may be applied continuously at relatively large timescales or pulsed sufficiently rapidly so as to form the Child-Langmuirsheath to reach the steady state sheath condition.

After the Child-Langmuir sheath is formed under biasing conditions, thesteady state sheath voltage V_(DC) causes the ions of all species tobombard the substrate with energy near eV_(DC) gained in the sheath(e.g. with some energies smaller due to possible collisions within thesheath). That is, rather than a significant spread of ion bombardmentenergy due to variation in start position of the ions, substantially allof the ions reaching the substrate are drawn into the Child-Langmuirsheath from the bulk plasma and fully accelerated by the sheath voltageV_(DC) to reach the substrate with energy eV_(DC). Further, as a resultof the relatively large timescales, both A⁺ and B⁺ ions may reach thesubstrate at approximately the same energy (e.g. in absence ofcollisions in the sheath).

Yet the different species in the plasma may serve different purposes.For instance, species A may be a precursor gas and species B may be aninert gas. In some cases, preferentially providing ions of one species(e.g. less reactive ions B⁺) to the substrate while decreasing orpreventing ions of other species (e.g. more reactive ions A⁺) fromreaching the substrate may be beneficial. However, conventional plasmaprocesses generally operate on timescales that form a voltage sheath atthe interface between the plasma and a substrate. Therefore,conventional plasma processes cannot significantly differentiate theenergies gained in the sheath for the ions of species A and B or theirflux compositions.

The embodiment methods and plasma processing apparatuses describedherein provide for selective separation of ions with different masses inthe plasma sheath to facilitate selective ion bombardment (i.e.preferential energetic bombardment by ions of a targeted mass) from asingle plasma volume. The selective separation and bombardment mayadvantageously enable advanced process control and improvements inprocess flexibility. For example, the embodiment methods and plasmaprocessing apparatuses may beneficially minimize surface bombardment byenergetic reactive species through appropriate process chemistryselection thereby leading to greater process selectivities.

Prior to the steady state formation of the sheath in response toexternal biasing conditions (e.g. external bias voltage V_(DC)), theinterface between a plasma and electrode changes dynamically from a wallsheath (e.g. when not voltage is applied at a bias electrode) to the ionmatrix and, finally to a Child law sheath. This dynamic intermediatephase represented by timescales shorter than the reciprocal plasmafrequency of the ions (t<ω_(pi) ⁻¹) is characterized by an expandingmatrix sheath that has been evacuated of electrons. Specifically, theinitial application of negative voltage to the bias electrode repels theelectrons from the region near the electrode leaving the ions in amatrix (e.g. because of the much greater mobility of electrons ascompared to more massive ions).

In the so-called matrix sheath regime (e.g. times in the rangeo<t<ω_(pi) ⁻¹), ions do not have enough time to fully traverse the widthof the sheath. However, analogous to differences in mobility betweenmassive ions and electrons, lighter (i.e. less massive) ion species havehigher mobility and travel faster within the matrix sheath than heavier(i.e. more massive) ion species. Consequently, the less massive ionsgain higher velocity, travel longer distances, passing through a higherpotential difference. This can temporarily lead to higher ion energiesand ion fluxes of the less massive ions at the bias electrode. At largertimescales (e.g. t>ω_(pi) ⁻¹), ions of all masses have sufficient timeto reach the substrate and the matrix sheath continues to expand until asteady state Child-Langmuir sheath is formed (t»ω_(pi) ⁻¹).

It should be noted that reference to ω_(pi) ⁻¹ herein is intended todescribe a qualitative timescale since the specific value is dependenton the ion mass. For the purposes of this disclosure, the lower bound ofω_(pi) ⁻¹ calculated using the least massive ion species can be assumedfor the inequality t<ω_(pi) ⁻1 while the upper bound calculated usingthe most massive ion species can be assumed for greater thaninequalities such as t»ω_(pi) ⁻¹.

The embodiment methods and plasma processing apparatuses describedherein generate plasma containing a first species and a second speciesthat is less massive than the first species. The first species ions andsecond species ions are spatially stratified in the plasma sheath byapplying a negative bias pulse to a bias electrode with pulse durationless than the reciprocal plasma frequency of the second species ions(e.g. less than 10 μs). For instance, in the matrix sheath regime, theflux and energy of ions of the second species at the substrate may beincreased relative to the flux and energy of ions of the first speciesat the substrate (e.g. to greater than 50%, greater than 67%, andhigher). The negative bias pulse is followed by a pulse delay duringwhich no bias voltage is applied to the bias electrode for at leastthree times the pulse duration.

In various embodiments, multiple negative bias pulses and correspondingpulse delays are applied to the bias electrode as a pulse train toselectively deliver second species ions to the bias electrode (e.g. asubstrate). The reactivity of the second species towards targetmaterials on the substrate may be different (e.g. lower) than thereactivity of the first species towards the target materials. Forexample, the first species may be a precursor gas while the secondspecies is an inert gas or both species may be precursor gases. Thepulse train may be applied using a short pulse generator coupled to thebias electrode.

Embodiment methods and plasma processing apparatuses may advantageouslyprovide various benefits over conventional methods and apparatuses.Short negative DC bias pulses applied to a bias electrode (e.g. asubstrate) may advantageously achieve preferable bombardment of thesubstrate by lighter ions. This preferable bombardment has theadditional advantage of not requiring any physical segregation ofspecies within the plasma processing chamber (i.e. all species are fullymixed in a single volume simultaneously). In contrast, selectivesegregation of the ions at the electrode is advantageously achievedusing differences in mobility due to differences in mass of the ions.Further, the ability to selectively provide a particular ion species atthe electrode may be beneficially leveraged to selectively deliver ionsof particular reactivity to the electrode (e.g. lessreactive/non-reactive vs. more reactive).

The increased ability to control ions at the substrate mayadvantageously enable enhanced controllability and flexibility in newprocesses. For example, the benefit of improving etch processselectivity for various target materials may be achieved by reducingsurface bombardment of reactive ions. A more massive species mayadvantageously be used as a source of reactive radicals while a lessmassive species may be used as a source of less reactive or non-reactiveions (e.g. substantially chemically neutral relative to substratesurface).

Additional flexibility in ion densities within the plasma may be enabledby increased ability to segregate ion species at the substrate. Forinstance, higher concentrations of reactive species may advantageouslybe included in the plasma processing chamber to increase the number ofreactive radicals at the substrate while limiting concentrations ofreactive ions. The matrix sheath current may also advantageously behigher than the steady state Child-Langmuir sheath current during thesheath transition.

Embodiments provided below describe various apparatuses and methods forplasma processing, and in particular, apparatuses and methods for plasmaprocessing in which ions of differing mass are spatially stratified inthe plasma sheath. The following description describes the embodiments.FIGS. 1A, 1B, and 1C are used to describe an embodiment plasma in thepresence of a bias electrode. FIG. 2 is used to describe an embodimentplasma processing apparatus. Three embodiment pulse trains are describedusing FIGS. 3-5. An embodiment continuous wave pulse train is describedusing FIG. 6 while an embodiment modulated wave pulse train is describedusing FIG. 7. Another embodiment plasma processing apparatus isdescribed using FIG. 8. Two embodiment methods are described using FIGS.9 and 10.

FIGS. 1A, 1B, and 1C schematically illustrate an example plasma in thepresence of a bias electrode in accordance with an embodiment of theinvention. FIG. 1A shows a wall sheath of the plasma interfacing withthe bias electrode when no voltage is applied. FIG. 1B shows a matrixsheath of the plasma interfacing with the bias electrode when a pulsedvoltage is applied. FIG. 1C shows a Child-Langmuir sheath of the plasmainterfacing with the bias electrode when continuous DC voltage isapplied.

Referring to FIGS. 1A, 1B, and 1C, a plasma 110 interfacing with a biaselectrode 115 includes two species (species A and species B). Species A(e.g. the first species) is more massive than species B (e.g. the secondspecies). Species A and B may also have different reactivity. That is,species A and B differ chemically and therefore may have differentreactivities toward various materials included in a target substrate.

The plasma 110 includes ions A⁺ and radicals A^(•) generated fromspecies A and ions B⁺ and radicals B^(•) generated from species B aswell as electrons 17. Since the ions A⁺ and B⁺ are created due to theloss of one or more electrons (i.e. negligible mass), the massrelationship of ions B⁺ being less massive than ions A⁺ is maintained.The ions A⁺ and B⁺ are positively charged while the radicals A^(•) andB^(•) have no net electric charge. Further, the plasma 110 may includevarious additional dissociation products, additional precursors, carrier(e.g. inert, buffer) gases, additives, negatively charged ionic species,and others (not shown). Although present, source species A and B are notshown in the plasma 110 for clarity.

Referring now specifically to FIG. 1A, no external voltage is applied tothe bias electrode 115 the voltage drop across the plasma sheath of theplasma 110 is small and abrupt resulting in a wall sheath 11. The wallsheath 11 is very thin having, for example, negligible sheath thickness(e.g. a couple of times the Debye length λ_(D)) in comparison to thesheath thickness in the presence of an applied voltage.

As shown in FIG. 1B, when a negative pulsed voltage −V_(P) is applied tothe bias electrode 115, the electrons 17 are immediately repelled by thenegative potential (−V_(P)) and move away from the bias electrode 115expanding the plasma sheath to form a matrix sheath 112. For example, attimescales on the order of the reciprocal electron plasma frequencyω_(pe) ⁻¹, the ions A⁺ and B⁺ have moved a negligible distance due toV_(P) while the electrons 17 have moved out of the matrix sheath 112.The matrix sheath 112 has a matrix sheath thickness s_(o) as measuredfrom the bias electrode 115.

At timescales larger than ω_(pe) ⁻¹ the ions A⁺ and B⁺ are depleted fromthe sheath as they reach the bias electrode 115 and the matrix sheath112 expands outward from the bias electrode 115 (illustrated by arrow19). The voltage drop across the matrix sheath 112 is the differencebetween the plasma potential and the bias electrode potential: thesheath potential V_(DC). Although the negative pulsed potential V_(P)and the sheath potential V_(DC) may be similar in many circumstances,they may also differ depending on the plasma potential.

For timescales of sufficient duration (i.e. t»ω_(pi) ⁻¹), the plasmasheath becomes a Child-Langmuir sheath 13 as shown in FIG. 1C. Forexample, a continuous negative DC voltage −V_(CW) is applied to the biaselectrode 115. That is, the magnitude of V_(P) and V_(CW) are notdifferent, but the duration of the applied voltage is differentresulting in the two regimes (matrix sheath regime vs. Child lawregime). The voltage −V_(CW) is continuous in the sense that it issufficiently long to allow the plasma sheath to reach a steady state.

The Child-Langmuir sheath 13 has a Child-Langmuir sheath thickness s_(C)as measured from the bias electrode 115. The matrix sheath thicknesss_(o) is smaller than s_(C) (e.g. five times smaller). For example, thematrix sheath thickness may be on the order of a couple of millimeters(e.g. 2.3 mm).

In contrast, as shown in FIG. 1B, for times t<ω_(pi) ⁻¹ (e.g. the matrixsheath regime) the interface between the plasma 110 and the biaselectrode 115 is highly dynamic. Initially, at t=ω_(pe) ⁻¹ the electrondensity in the matrix sheath 112 can be considered zero (n_(e)=o) whilethe ion density n_(i) is constant because the ions A⁺ and B⁺ move muchmore slowly than the electrons 17.

Following the initial creation of the matrix sheath 112, the exposedions A⁺ and B⁺ are drained from the matrix sheath 112 to the biaselectrode 115. The potential within the matrix sheath 112 behavesquadratically. The ion energy distribution E_(i) can be expressed by thepotential distribution in the matrix sheath 112.

Each of the ions A⁺ and B⁺ arrive at the bias electrode 115 with abombardment energy dependent on starting position within the matrixsheath 112. Specifically, if x is the starting position of an ion, theion will arrive with energy

$e{V_{DC}\left\lbrack {1 - \frac{\left( {s_{0} - x} \right)^{2}}{s_{0}^{2}}} \right\rbrack}$

Hence, the bombardment energy of all the ions A⁺ and B⁺ is less thaneV_(DC) since the time t is less than the time it takes for either ionspecies to fully traverse the matrix sheath 112.

Due to the difference in ion mass between ions A⁺ and B⁺, the ions crossthe plasma sheath at different velocities. The ion velocity (υ) throughthe plasma sheath s_(o) may be represented as:

$\begin{matrix}{{v(x)} = {\sqrt{\frac{2eV_{DC}}{M_{i}}}\left\lbrack {1 - \frac{\left( {s_{0} - x} \right)^{2}}{s_{0}^{2}}} \right\rbrack}^{1/2}} & (x)\end{matrix}$

The ion velocity also depends on the ion mass, M_(i). Hence, the ionswith lighter mass gain higher velocity through the plasma sheath thanions with heavier mass. Traveling longer distances and over a greaterpotential difference results in higher energies. As the result thelighter ions B⁺ gain more energy and travel greater distances in ashorter time compared to the heavier ions A⁺ (i.e. when both start atthe same distance from the electrode).

As illustrated by A⁺ travel distance d_(A) and B⁺ travel distance d_(B),the lighter ions B⁺ travel a greater distance in the same amount oftime. Due to the increased mass of species A relative to species B,d_(B)>d_(A). The difference between d_(A) and d_(B) increases withincreased mass disparity. For example, for helium (He) and chlorine(Cl₂) which have masses of 4 amu (atomic mass units) and 70.9 amurespectively, an He⁺ ion (B⁺) may move about three times as far as a Cl₂⁺ ion (A⁺). However ratio of d_(B):d_(A) may be greater than or lessthan 3:1 (but always greater than 1:1) depending on the particularmasses involved.

A further implication is that the less massive ions B⁺ have highermobility compared to the more massive ions A⁺. The ions A⁺ and B⁺ in thematrix sheath 112 are depleted at different rates due to the differencein mobility. Therefore, the location of peak current will occur at adifferent time for ions of different masses (e.g. at ˜50 ns for He⁺). Inthis way, the creation of the matrix sheath 112 advantageously separatesions of different masses (ions A⁺ and B⁺) in the plasma sheath for timesless than or comparable with the B⁺ ion transit time through the sheath.

The current density j_(P) in the matrix sheath 112 is dynamic and higherthan the substantially constant Child-Langmuir current density j_(C).For example, after the ions begin to move out of the matrix sheath 112j_(P) rapidly increases, peaks around t≈ω_(pi) ⁻¹, and then rapidlydecreases to a steady state in the Child law regime (j_(C)). As aconsequence, j_(p) may have the advantage of being higher than j_(C)(e.g. 5 to 10 times higher or more).

At timescales shorter than ω_(pi) ⁻¹ for the lightest species, thedisparity between the concentration of the ions A⁺ and B⁺ at the biaselectrode 115 may advantageously be maximized since the current densityof the B⁺ ions peaks but the current density of the A⁺ is far frompeaking.

FIG. 2 schematically illustrates an example plasma processing apparatusincluding a matrix sheath of a plasma interfacing with a substratewithin a plasma processing chamber, the substrate being coupled to ashort pulse generator in accordance with an embodiment of the invention.The plasma processing apparatus of FIG. 2 may be used to generateembodiment plasmas as described herein, such as the plasma of FIGS. 1A,1B, and 1C, for example. Similarly labeled elements may be as previouslydescribed.

Referring to FIG. 2, a plasma processing apparatus 200 includes plasma210 contained within a plasma processing chamber 220. It should be notedthat here and in the following a convention has been adopted for brevityand clarity wherein elements adhering to the pattern [x10] may berelated implementations of plasma in various embodiments. For example,the plasma 210 may be similar to the plasma 110 except as otherwisestated. An analogous convention has also been adopted for other elementsas made clear by the use of similar terms in conjunction with theaforementioned three-digit numbering system.

The plasma 210 includes a matrix sheath 212 generated by applying a biasvoltage in the form of a negative pulsed voltage −V_(P) delivered to asubstrate 215 which may be a specific implementation of a bias electrode(e.g. immobilized by a substrate holder acting as a bias electrode). Thesubstrate 215 includes a surface 216 exposed to the plasma 210 whichinterfaces with the substrate 215 at the matrix sheath 212. For example,the surface 216 is an etchable surface in one embodiment.

The negative pulsed voltage −V_(P) is generated by a short pulsegenerator 222 coupled to the substrate 215 (e.g. via a substrateholder). As shown, the short pulse generator delivers at least onenegative voltage pulse of duration less than the reciprocal ion plasmafrequency (t<ω_(pi) ⁻¹). The negative voltage pulse is followed by apulse delay that of sufficient duration to allow the plasma 210 recover(e.g. return to a wall sheath state). In various embodiments, thenegative voltage pulse and pulse delay are cyclically repeated as apulse train of appropriate length for the desired processing of thesurface 216.

Still referring to FIG. 2, the plasma 210 as shown includes species Aand species B as well as corresponding ions A⁺ and B⁺ and radicals A^(•)and B^(•). As before, many other species such as dissociation products,additives, etc. may also be present in the plasma 201. In variousembodiments, the species B and its derivatives are less reactive thanspecies A and its derivatives. For this reason species A may sometimesbe referred to as a reactive species while species B may be referred toas a non-reactive species herein. However, it is also possible forspecies B to be more reactive than species A when reactive ions aredesired at the substrate 215.

In various embodiments species A is a precursor gas and istetrafluoromethane (carbon tetrafluoride, CF₄) in one embodiment.Species A may also be a hydrofluorocarbon (C_(x)H_(y)F_(z)) in variousembodiments. In another embodiment, species A is chlorine gas (Cl₂). Instill another embodiment, species A is hydrogen bromide (HBr). Ofcourse, other precursors are also possible and may depend on thespecific details of the desired plasma process being performed. In somecases a heavy inert gas may be used for species A, such as if reactiveions are desired at the substrate 215.

Species B may be an inert gas or precursor gas that is less massive thanspecies A. In some embodiments, species B is an inert gas and is heliumgas (He) in one embodiment. Other possible inert gases may also be usedas species B such as neon gas (Ne), argon gas (Ar), and others (althoughincreased species B mass limits available more massive species for A).In other embodiments, species B is a precursor gas. In one embodiment,species B is hydrogen gas (H₂). In another embodiment, species B isoxygen gas (O₂).

The negative bias voltage −V_(P) spatially stratifies the ions A⁺ and B⁺in the plasma sheath (matrix sheath 212). For example, as illustrated byqualitative stratification line 18, at a time t<ω_(pi) ⁻¹ the B⁺ ionshave moved to the bottom of the matrix sheath 212 while the A⁺ remain atthe top of the matrix sheath 212. In this way, a small number of A⁺ ionsthat were initially close to the substrate 215 impinge on the surface216 while a large number of B⁺ ions reach the surface 216. The flux ofB⁺ ions is therefore advantageously increased at the substrate 215.

The electrically neutral species A, B, A^(•), and B^(•) are unaffectedby the applied voltage −V_(P). Consequently, the concentration ofradicals A^(•) and B^(•) at the substrate 215 remains the same. Forexample, species A may be a etchant source with the A^(•) radicals beingdesirable as etchants at the surface 216 while reactive A⁺ ions areundesirable at the surface 216. The increased flux of B⁺ ions at thesubstrate 215 preferentially bombards the surface 216 with B⁺ ions at ahigher energy than the A⁺ ions.

This, in turn, may advantageously increase selectivity in etchprocesses. For many processes, selectivity may be defined as a ratio ofthe flux of reactive radicals to the flux of reactive ions.Consequently, surface bombardment by reactive ions may be unwanted as itmay result in selectivity loss. Species A, with greater mass (i.e.heavier molecular/atomic weight) can be used as a source of reactiveradicals whereas species B can be used as the source of non-reactiveions.

Electric field modulation in the plasma sheath by short bias pulses mayadvantageously lead to preferable surface bombardment by more energeticnon-reactive ions B⁺ while electrically neutral reactive radicals A^(•)are used as the main etchant improving the etch selectivity.

In various embodiments, a plasma etching process includes ahalogen-based precursor gas as species A and an inert gas as species B.Unwanted surface bombardment by halogen ions may be reduced oreliminated by supplying short pulse biases to a substrate. In oneembodiment, the species A is CF₄ and species B is an inert gas. Forexample, the inert gas may be He, Ar, and others. CF_(x) ⁺ ionbombardment may be advantageously minimized while inert ions (e.g. He⁺ions) may perform ion assisted etching in the fluoro-containing plasma.

Many other possible combinations of halogen-based precursor gases andinert gases exist. In another embodiment, species A is CL₂ and species Bis He. In still other embodiments, species A is hydrogen bromide (HBr)while species B is an inert gas such as He or Ar.

Species A and species B may also both be precursor gases (e.g. with orwithout an additional inert gas). In various embodiments, species A is ahalogen-based precursor gas while species B is a non-halogen-basedprecursor gas. For example, in one embodiment, species A is hydrogenchloride (HCl) and species B is hydrogen gas (H₂). In this example,bombardment of a substrate by H⁺ ions may be preferred (e.g. for ionassisted etching).

In C_(x)H_(y)F_(z) plasmas, the carbon to fluoride ratio (C:F) may bealtered be appropriate selection of x, y, and z as well as withadditives such as O₂ or H₂. Several species of different masses may bepresent in such a system. It may be advantageous to limit unwantedsurface bombardment by fluorine-containing species (e.g. C_(x)F_(y)).Segregation of ions of different mass is not limited to binary cases ofspecies A and species B, but may be extended to include as many speciesas may be in a given plasma. For example, a C_(x)H_(y)F_(z) plasma mayinclude several less-massive non-halogen-based species that may all bepreferentially delivered to a substrate surface over fluorine-containingspecies.

It should be noted that although the examples herein refer to thepreferential bombardment of a surface by higher energy ions of singlelighter species relative to a single heavier ionic species, more thantwo ions may be included. For example, ions A⁺, B⁺, C⁺, etc. havingdifferent masses may be included and the least massive ions B⁺ withhigher energy may preferentially bombard the surface. Further, a subset(e.g. one or more) of the ions may be targeted to preferentially bombardthe surface with higher energy. For instance, the negative bias voltageV_(P) may be applied for a sufficient amount of time to generate anincrease in flux of the two lightest ionic species of several ionicspecies.

FIGS. 3-7 illustrate various pulse trains which may be applied to a biaselectrode in order to generate a plasma with a matrix sheath andselectively segregate ions of differing mass from one another. The pulsetrains of FIGS. 3-7 may be used to generate plasma using plasmaprocessing apparatuses as described here such as the plasma processingapparatus of FIG. 2, for example. Similarly labeled elements may be aspreviously described.

The various pulse trains include negative bias pulses with a pulseduration t_(P) sufficiently short so as to prevent Child law plasmabehavior (t_(P)<ω_(pi) ⁻¹). In various embodiments, the pulse durationt_(P) is in the range of tens of nanoseconds to tens of microseconds. Insome embodiments, the pulse duration t_(P) is less than about 100 ns andis about 75 ns in one embodiment. The specific values of the pulseduration t_(P) will depend in the operating conditions of a specificimplementation. In one embodiment, the negative bias pulses are DC biaspulses. Alternatively, the pulse trains may be generated by modulatingRF power.

Each negative bias pulse is followed by a corresponding pulse delayt_(off) during which no bias voltage is applied to the bias electrodeand of sufficient length (t_(off)»ω_(pi) ⁻¹) to allow the plasma torecover from the negative bias pulse (e.g. regain a substantiallyconsistent distribution of species in the plasma and form a wallsheath). The pulse delay t_(off) is several multiples of t_(P) in length(e.g. 3, 5, and higher). In various embodiments, the pulse delay t_(off)is greater than about 150 ns and is about 375 ns in one embodiment. Inother embodiments, the pulse delay t_(off) may range from tens ofnanoseconds to milliseconds.

Each of the negative bias pulses also has a pulse amplitude V_(P) thatalters the voltage drop across the plasma sheath and accelerates ionstowards the bias electrode. The pulse amplitude V_(P) may be anysuitable voltage as may be desirable for a given plasma process andtarget substrate. In various embodiments, the pulse amplitude V_(P) isless than about 500 V and is between about 25 V and about 150 V in someembodiments. In one embodiment, the pulse amplitude V_(P) is about 50 V.In another embodiment, the pulse amplitude V_(P) is about 100 V.

FIG. 3 illustrates an example pulse train of negative bias pulses inaccordance with an embodiment of the invention. Referring to FIG. 3, apulse train 330 includes negative bias pulses 332 with pulse duration334 (t_(P)) and pulse amplitude 338 (V_(P)) followed by a pulse delay336 (t_(off)). The pulse train 330 includes a period 339, as shown. Theapplication of a negative bias pulse followed by a pulse delay isrepeated cyclically as needed. Although the pulse duration 334 and thepulse delay 336 are illustrated as remaining constant, it is alsopossible to dynamically vary these parameters while still maintainingthe relationship with the ion plasma frequency (t_(P)<ω_(pi)⁻¹«t_(off)).

FIG. 4 illustrates an example pulse train of negative bias pulse andpositive bias pulse in accordance with an embodiment of the invention.Referring to FIG. 4, a pulse train 430 includes negative bias pulses 432with negative pulse duration 434 (t_(P)) and negative pulse amplitude438 (V_(P)) followed by a pulse delay 436 (t_(off)). The pulse train 430includes a period 439, as shown. Additionally, the pulse train 430 alsoincludes positive bias pulse 442 with positive pulse duration 444 andpositive pulse amplitude 448.

The positive bias pulses 442 may be used to advantageously impart aforce on the plasma electrons directed toward the bias electrode forfaster sheath recovery after each of the negative bias pulses 432. Boththe positive pulse duration 444 and positive pulse amplitude 448 may besmaller than the negative pulse duration 434 and the negative pulseamplitude 438 as shown, but this is not a requirement. In some cases thepositive pulse duration 444 and positive pulse amplitude 448 may beappropriately chosen in order to advantageously accelerate electronsfrom the bulk plasma into the matrix sheath to allow faster recovery ofthe sheath.

FIG. 5 illustrates an example pulse train of negative bias pulses, eachwith a linear voltage slope in accordance with an embodiment of theinvention. Referring to FIG. 5, a pulse train 530 includes negative biaspulses 532 with pulse duration 534 (t_(P)) and pulse amplitude 538(V_(P)) followed by a pulse delay 536 (t_(off)). The pulse train 530includes a period 539, as shown. In contrast to the square wave shape ofthe negative bias pulses of FIG. 3, the negative bias pulses 532 of FIG.5 include a linear voltage slope 52 which may have the advantage ofreducing or minimizing adverse charging effects at the bias electrode(e.g. a substrate including dielectric material).

FIG. 6 illustrates an example continuous wave pulse train includingnegative bias pulses and positive bias pulses in accordance with anembodiment of the invention. Referring to FIG. 6, a continuous wavepulse train 630 includes negative bias pulses 632 and positive biaspulses 642 followed by a pulse delay. As shown, the application of anegative bias pulse followed by a pulse delay is repeated cyclicallywith period 639 for many cycles without disruption. The continuous wavepulse train 630 may be a specific implementation of other pulse trainsdescribed herein such as the pulse train 330 of FIG. 3 as well as thepulse trains of FIGS. 4-5.

FIG. 7 illustrates an example modulated wave pulse train including asurface reaction phase followed by a chemical modification phase inaccordance with an embodiment of the invention. Referring to FIG. 7, amodulated wave pulse train 730 includes negative bias pulses 732 andpositive bias pulses 742 followed by a pulse delay repeated cyclicallywith period 739. Different from the continuous wave pulse train 630 ofFIG. 6, the modulated wave pulse train 730 includes a surface reactionphase 84 followed by a chemical modification phase 86 during which nopulses are generated.

The surface reaction phase 84 may leverage the relatively shorttimescale of surface reactions to process the surface of a substrateduring negative bias pulses. In contrast, the chemical modificationphase 86 may be sufficiently long so as to allow the relatively longtimescale of surface restoration processes to take effect. For example,the chemical modification phase 86 may be a chemistry adsorption phase.

It should be noted that some or all of the features described usingFIGS. 3-7 may be combined. For example, both positive bias pulses andnegative bias pulse with a linear voltage slope may be implemented inthe same pulse train generated by modulating RF power. Further, asurface reaction phase may be included in a pulse train utilizing alinear voltage slope but not implementing positive bias pulses. Othercombinations will be apparent to those of skill in the art in view ofthe totality of the disclosure.

FIG. 8 schematically illustrates an example plasma processing apparatusincluding a plasma coupling element used to generate a plasma, a firstgas source, a second gas source, and a substrate within a plasmaprocessing chamber, the plasma including a matrix sheath interfacingwith the substrate which is coupled to a short pulse generator inaccordance with an embodiment of the invention. The plasma processingapparatus of FIG. 8 may be a specific implementation of other plasmaprocessing apparatuses described herein such as the plasma processingapparatus of FIG. 2, for example. Similarly labeled elements may be aspreviously described.

Referring to FIG. 8, a plasma processing apparatus 800 includes plasma810 contained within a plasma processing chamber 820. The plasma 810 isgenerated using an RF power source 90 coupled to a plasma couplingelement 92 and includes a matrix sheath 812, the plasma 810 with thematrix sheath 812 being similar to other plasmas described herein. Anegative pulsed voltage V_(P) is applied to a substrate 815 by way of ashort pulse generator 822 coupled to the substrate 815 and supplied by abias power source 98.

The plasma 810 may an RF plasma as illustrated or may be any othersuitable type of plasma. For example, the plasma 810 may be acapacitively coupled plasma (CCP), an inductively coupled plasma (ICP),a surface wave plasma (SWP), electron cyclotron resonance (ECR) plasma,helical resonator (HR) plasma, and others. The specific implementationof the plasma coupling element 92 may depend on the plasma 810. In oneembodiment, the plasma 810 is a CCP plasma and the plasma couplingelement 92 is an upper electrode. In other embodiments, the plasma 810is an ICP plasma and the plasma coupling element 92 is a coil or anantenna that couples source power to the plasma 810 through a dielectricmaterial of the plasma processing chamber 820.

As before, the plasma 810 includes at least two species: A and B whichmay be delivered using a species A source gas 94 and a species B sourcegas 95 (e.g. through a showerhead at the top of the plasma processingchamber 820 or through other suitable means). For example, gas inletsmay also be included in walls of the plasma processing chamber 820. Insome embodiments, the plasma coupling element 92 may also be ashowerhead with multiple gas inlets to even distribute the delivery ofspecies A and species B into the plasma processing chamber 820.Alternatively, (e.g. for ICP plasmas), a dielectric gas delivery systemmay also be used.

The gas within the plasma processing chamber 820 is evacuated using oneor more vacuum pumps 96, such as a single stage pumping system or amultistage pumping system (e.g. a mechanical roughing pump combined withone or more turbomolecular pumps). For example, vacuum pumps 96 may beconfigured to remove gas from the plasma processing chamber 820 throughon or more gas outlets 97. In order to promote even gas flow duringplasma processing, gas may be removed from more than one gas outlet orlocation in the plasma processing chamber 820 (e.g. on opposite sides ofthe substrate 815, a ring around the substrate 815, etc.)

The pressure within the plasma processing chamber 820 may be controlledusing gas flowrates of the species A source gas 94 and species B sourcegas 95 while gas is pumped out of the system using the vacuum pumps 96.Absolute and relative ion densities of the different species (A, B,etc.) affect the concentration of reactive ions at the substrate. Theion densities may advantageously be controlled using gas flowrates ofthe species A source gas 94 and species B source gas 95 to furtherimprove selectivity of a given plasma process.

FIG. 9 illustrates an example method of plasma processing in accordancewith an embodiment of the invention. The method of FIG. 9 may beperformed using the plasma processing apparatuses, plasmas, and pulsetrains as described herein. For example, the method of FIG. 9 may becombined with any of the embodiments of FIGS. 1-8. Although shown in alogical order, the arrangement and numbering of the steps of FIG. 9 arenot intended to be limited. The method steps of FIG. 9 may be performedin any suitable order or concurrently with one another as may beapparent to a person of skill in the art.

Referring to FIG. 9, step 901 of a method 900 of plasma processing is togenerate plasma in a plasma processing chamber. The plasma processingchamber contains a first species, a second species, and a substrate. Theplasma generated in the plasma processing chamber includes a plasmasheath, first species ions, and second species ions. The first specieshas a first mass and the second species has a second mass less than thefirst mass.

Step 902 is to apply a pulse train of negative bias pulses to thesubstrate. Each of the negative bias pulses spatially stratifies thefirst species ions and the second species ions in the plasma sheath. Theduration of each of the negative bias pulses is less than 10 μs invarious embodiments and is less than 1 μs in some embodiments. No biasvoltage is applied to the substrate during a pulse delay after eachnegative bias pulse. In various embodiments, the pulse delay is at leastthree times the pulse duration and is at least five times the pulseduration in some embodiments.

FIG. 10 illustrates another example method of plasma processing inaccordance with an embodiment of the invention. The method of FIG. 10may be performed using the plasma processing apparatuses, plasmas, andpulse trains as described herein. For example, the method of FIG. 10 maybe combined with any of the embodiments of FIGS. 1-8. Additionally, themethod of FIG. 10 may be combined with other methods such as the methodof FIG. 9, for example. Although shown in a logical order, thearrangement and numbering of the steps of FIG. 10 are not intended to belimited. The method steps of FIG. 10 may be performed in any suitableorder or concurrently with one another as may be apparent to a person ofskill in the art.

Referring to FIG. 10, step 1001 of a method 1000 of plasma processing isto generate plasma in a plasma processing chamber containing aless-reactive species, a more-reactive species, and a substrate thatincludes an etchable surface. The plasma includes ions of theless-reactive species, and ions and radicals of the more-reactivespecies. The mass of the less-reactive species is less than the mass ofthe more-reactive species. The reactivity of the less-reactive speciestowards the etchable surface is less than the reactivity of themore-reactive species towards the etchable surface.

Step 1002 is to increase the flux and energy of the ions of theless-reactive species at the substrate relative to the flux and energyof the ions of the more-reactive species at the substrate by applying apulse train of negative bias pulses to the substrate. The duration ofeach of the negative bias pulses is less than 10 μs in variousembodiments. The etchable surface of the substrate is etched usingradicals of the more-reactive species in step 1003.

Example embodiments of the invention are summarized here. Otherembodiments can also be understood from the entirety of thespecification as well as the claims filed herein.

EXAMPLE 1

A method of plasma processing including: generating plasma in a plasmaprocessing chamber containing a first species, a second species, and asubstrate, the plasma including a plasma sheath, first species ions, andsecond species ions, where the first species has a first mass and thesecond species has a second mass less than the first mass; and applyinga pulse train of negative bias pulses to the substrate, where each ofthe negative bias pulses has a pulse duration less than 10 μs andspatially stratifies the first species ions and the second species ionsin the plasma sheath, and where no bias voltage is applied to thesubstrate during a pulse delay after each negative bias pulse, the pulsedelay being at least five times the pulse duration.

EXAMPLE 2

The method of example 1, where the first species is a precursor gas andthe second species is an inert gas.

EXAMPLE 3

The method of one of examples 1 and 2, where the first species is afirst precursor gas and the second species is a second precursor gas.

EXAMPLE 4

The method of one of examples 1 to 3, where the pulse duration is lessthan about 250 ns.

EXAMPLE 5

The method of one of examples 1 to 4, where the pulse delay is greaterthan about 10 μs.

EXAMPLE 6

The method of one of examples 1 to 5, where the reactivity of the firstspecies towards an etchable surface of the substrate is greater than thereactivity of the second species towards the etchable surface of thesubstrate.

EXAMPLE 7

The method of one of examples 1 to 6, further including: etching thesubstrate using first species radicals.

EXAMPLE 8

The method of one of examples 1 to 7, where repeating the steps furtherincludes: applying a positive bias pulse after each of the negative biaspulses and before each pulse delay.

EXAMPLE 9

The method of one of examples 1 to 8, where a leading edge of each ofthe negative bias pulses includes a linear voltage slope.

EXAMPLE 10

The method of one of examples 1 to 9, further including: after applyingthe pulse train for a first duration, performing a chemical modificationphase by applying no bias voltage to the substrate for a second durationgreater than ten times the sum of the pulse duration and the pulsedelay; and after the chemical modification phase, resume applying thepulse train for a third duration.

EXAMPLE 11

A method of plasma processing including: generating plasma in a plasmaprocessing chamber containing a less-reactive species, a more-reactivespecies, and a substrate including an etchable surface, the plasmaincluding ions of the less-reactive species, and ions and radicals ofthe more-reactive species, where the mass of the less-reactive speciesis less than the mass of the more-reactive species, and where thereactivity of the less-reactive species towards the etchable surface isless than the reactivity of the more-reactive species towards theetchable surface; increasing the flux and energy of the ions of theless-reactive species at the substrate relative to the flux and energyof the ions of the more-reactive species at the substrate by applying apulse train of negative bias pulses to the substrate, each negative biaspulse having a pulse duration less than 10 μs; and etching the etchablesurface of the substrate using the radicals of the more-reactivespecies.

EXAMPLE 12

The method of example 11, where the less-reactive species is an inertgas and the more-reactive species is a precursor gas.

EXAMPLE 13

The method of example 12, where the inert gas is helium and theprecursor gas is a fluorocarbon.

EXAMPLE 14

The method of one of examples 11 to 13, where the less-reactive speciesis a first precursor gas and the more-reactive species is a secondprecursor gas.

EXAMPLE 15

The method of example 14, where the first precursor gas is hydrogen (H₂)and the second precursor gas is hydrogen chloride (HCl).

EXAMPLE 16

The method of one of examples 11 to 15, where a pulse delay betweensuccessive negative bias pulses is at least five times the pulseduration.

EXAMPLE 17

A plasma processing apparatus including: a plasma processing chamberconfigured to contain a plasma including a plasma sheath, ions of afirst species, and ions of a second species, where the first species hasa first mass and the second species has a second mass less than thefirst mass; a substrate disposed in the plasma processing chamber; and ashort pulse generator coupled to the substrate, the short pulsegenerator configured to generate a pulse train of negative bias pulses,where each of the negative bias pulses has a pulse duration less than 10μs, where a pulse delay between successive negative bias pulses is atleast five times the pulse duration, and the pulse train spatiallystratifies the ions of the first species and the ions of the secondspecies in the plasma sheath.

EXAMPLE 18

The plasma processing apparatus of example 17, where the pulse durationis less than about 250 ns.

EXAMPLE 19

The plasma processing apparatus of one of examples 17 and 18, where thepulse delay is greater than about 10 μs.

EXAMPLE 20

The plasma processing apparatus of one of examples 17 to 19, where thepulse train further includes positive bias pulses immediately followingeach of the negative bias pulses, the pulse delay between successivenegative bias pulses immediately following each of the positive biaspulse.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A plasma processing apparatus comprising: aplasma processing chamber configured to contain a plasma comprising aplasma sheath, ions of a first species, and ions of a second species,wherein the first species has a first mass and the second species has asecond mass less than the first mass; a substrate disposed in the plasmaprocessing chamber; and a short pulse generator coupled to thesubstrate, the short pulse generator configured to generate a pulsetrain of negative bias pulses, wherein each of the negative bias pulseshas a pulse duration less than 10 μs, wherein a pulse delay betweensuccessive negative bias pulses is at least five times the pulseduration, and wherein the pulse train spatially stratifies the ions ofthe first species and the ions of the second species in the plasmasheath.
 2. The plasma processing apparatus of claim 1, wherein the pulseduration is less than about 250 ns.
 3. The plasma processing apparatusof claim 1, wherein the pulse delay is greater than about 10 μs.
 4. Theplasma processing apparatus of claim 1, wherein the pulse train furthercomprises positive bias pulses immediately following each of thenegative bias pulses, the pulse delay between successive negative biaspulses immediately following each of the positive bias pulse.
 5. Amethod of plasma processing comprising: generating plasma in a plasmaprocessing chamber containing a less-reactive species, a more-reactivespecies, and a substrate comprising an etchable surface, the plasmacomprising ions of the less-reactive species, and ions and radicals ofthe more-reactive species, wherein the mass of the less-reactive speciesis less than the mass of the more-reactive species, and wherein areactivity of the less-reactive species towards the etchable surface isless than a reactivity of the more-reactive species towards the etchablesurface; increasing the flux and energy of the ions of the less-reactivespecies at the substrate relative to the flux and energy of the ions ofthe more-reactive species at the substrate by applying a pulse train ofnegative bias pulses to the substrate, each negative bias pulse having apulse duration less than 10 μs; and etching the etchable surface of thesubstrate using the radicals of the more-reactive species.
 6. The methodof claim 5, wherein the less-reactive species is an inert gas and themore-reactive species is a precursor gas.
 7. The method of claim 6,wherein the inert gas is helium and the precursor gas is a fluorocarbon.8. The method of claim 5, wherein the less-reactive species is a firstprecursor gas and the more-reactive species is a second precursor gas.9. The method of claim 8, wherein the first precursor gas is hydrogen(H₂) and the second precursor gas is hydrogen chloride (HCl).
 10. Themethod of claim 5, wherein a pulse delay between successive negativebias pulses is at least five times the pulse duration.
 11. A plasmaprocessing apparatus comprising: a plasma processing chamber configuredto contain a plasma comprising a plasma sheath, ions of a first species,and ions of a second species, wherein the first species has a first massand the second species has a second mass less than the first mass; asubstrate disposed in the plasma processing chamber; and wherein theplasma processing apparatus is configured to generate a pulse train ofnegative bias pulses, each having a pulse duration less than 10 μs,wherein a pulse delay between successive negative bias pulses is atleast five times the pulse duration.
 12. The plasma processing apparatusof claim 11, wherein the pulse train spatially stratifies the ions ofthe first species and the ions of the second species in the plasmasheath.
 13. The plasma processing apparatus of claim 11, furthercomprising: a short pulse generator coupled to the substrate andconfigured to generate the pulse train of negative bias pulses.
 14. Theplasma processing apparatus of claim 13, wherein the short pulsegenerator is further configured to apply a positive bias pulse aftereach of the negative bias pulses and before each pulse delay.
 15. Theplasma processing apparatus of claim 14, wherein each applied positivebias pulse immediately follows the corresponding negative bias pulse.16. The plasma processing apparatus of claim 13, wherein a leading edgeof each of the negative bias pulses comprises a linear voltage slope.17. The plasma processing apparatus of claim 13, wherein the short pulsegenerator is further configured to after applying the pulse train for afirst duration, apply no bias voltage to the substrate for a secondduration greater than ten times the sum of the pulse duration and thepulse delay, and resume applying the pulse train after the secondduration.
 18. The plasma processing apparatus of claim 11, furthercomprising: an upper electrode configured to generate the plasma in theplasma processing chamber, the plasma being a capacitively coupledplasma.
 19. The plasma processing apparatus of claim 11, wherein thepulse duration is less than about 250 ns.
 20. The plasma processingapparatus of claim 11, wherein the pulse delay is greater than about 10μs.